Soya bean and its various products that are derived from soya beans contain anti nutritional factors. Some of these factors include residual anti-nutrients that reduce digestibility, protease inhibitors (anti-trypsin), allergenic proteins, lipozygenase, urease and lecitins.
In the manufacturing processes, soya needs to exposed to heat in order to denature these factors which would otherwise negatively affect performance.
While we know that under-heating of soya products leaves detrimental levels of anti-nutritional factors in the feed and GIT, over-heating can also have a deleterious effect. Over-heating reduces the digestibility of lysine and other nutrients. In the heating process the balance between under-heating and over-heating must be maintained.
In 1917 (Osborne and Meida) identified that the heat treatment of soya improved nutritional value.
In 1944 Carly and Knapp devised a method to detect poorly heated soya bean meal (SBM) by checking on the activity of a naturally occurring enzyme in soya called urease.
As a result of this observation, the Professional Chemists Association adapted a pH rise as a measure of urease activity in soya. A rise of between 0,05 and 0,2 pH points was accepted as a suitable result that showed soya was adequately heat-treated.
A change of less than 0,05 pH points was considered over-heated with resultant consequences on lysine digestibility.
A change of more than 0,2 pH points shows that there are still active anti-nutrient factors present in the soya. i.e. the urease enzyme is still variably active and is producing ammonia which results in the increased pH change.
Dale (1987) however demonstrated that low urease activity was inaccurate in determining over processing. A change of less than 0,05 pH does not confirm over heating.
In order to determine over-heating, protein solubility in KOH is the most accurate method. A reading of between 72% and 85% shows correct heating. Below 72% will indicate over heating and lysine digestibility issues.
On the upper readings of urease activity, changes of 0,3 to 0,5 pH have been considered acceptable when testing for adequately heated treated soya.
While the use of urease testing is quick, easy and accurate there is no biological relationship between urease activity and trypsin inhibition. In fact urease has no negative effect on monogastrics. The relationship is based purely on the fact that the heat required to denature urease coincidentally denatures anti trypsin factors. Urease activity is therefore only used as a marker of trypsin inhibitor activity.
Tables exist that show the relationship between trypsin inhibitors (TI) and urease activity (UA). (Mustches et al 1981). From the tables a UA reading above 0,05 pH indicates a trypsin activity above 1,6 mg/gm.
Since 2005 there has been positive association between a high TI reading and “rapid feed passage” (RFP). A TI above 3,5 mg/gm will lead to the observed condition of RFP.
Ideally one requires a TI reading below 2,0 mg/gm and a KOH protein solubility of above 72%. This correlates to a pH change of 0,05.
Most of the trails and data on soya quality relate back 50 years. A comparison between a 1957 broiler versus a 2001 broiler together with the nutrition of the day reveals an important variance in trypsin inhibitor intake.
Feeding a 1957 broiler with soya in which urease activity increased pH by 0,2 would provide a total intake over 42 days of 1200 mg of trypsin inhibitor.
Feeding a 2001 broiler with a soya in which urease activity increased pH of 0,05 would provide a total intake over 42 days of 2007 mg of trypsin inhibitor.
Based on the above facts, it is vital that the industry look more closely at modern day acceptable levels of UA. No longer can a UA of 0,3 pH be considered acceptable.
Latest thoughts (2012) are these changes are far too high and that the acceptable range be a pH change between 0,00 and 0,05 pH.
Other Test Methods
This refers to the test method for protein solubility. This test is used to determine if soya has been over-heat treated and the lysine digestibility has been detrimentally affected.
It is widely accepted that a reading below 72% indicates over-heating and reduced lysine activity. An ideal result of 72% to 83% is considered a normal acceptable range.
Protein Dispersibility Index (PDI)
PDI is used as an alternate test method to urease and KOH.
The correlation of 45-50% and less than 0,3 pH urease can be used to indicate correctly heat processed soya.